In this specification, by way of example only, the invention will be described with reference to the control of herbicidal sprays. However, it should be realised that this invention does have important applications in controlling the use of other agricultural sprays, such as fertilizers and insecticides and for detectors used to estimate the biomass of an area.
In minimum-tillage or non-tillage cultivation, herbicides are used to control unwanted weed growth in a field. The main reason for controlling unwanted growth is to minimize the loss of moisture and nutrients from the soil. The herbicides used in these types of cultivation are expensive and in many situations it is not necessary for the entire field to be sprayed as unwanted vegetation may not cover the entire area. However, previously, the entire field would be sprayed to ensure that all the weeds are sprayed, thus resulting in unnecessary spraying of some areas.
Recent developments have produced vegetation detection units which detect the amount of radiance in the red and near infra-red wavebands. These detection units have been used to provide an estimate of the biomass in a given area. The estimate provided is based on the difference in the measured ratios of the red to near infra-red radiance between an area of bare soil, used as a reference value, and the area under investigation. Satisfactory use of these radiation detectors depends upon the intensity of irradiance in the area and is thus less accurate during periods of changing light conditions. These vegetation detection units, in making an estimate of the percentage of biomass in the area have used a simple ratio of the radiance in the red and near infra-red wavebands of the electromagnetic spectrum.
Irradiance may be defined as the rate of flow of energy onto a surface, per unit surface area, and radiance may be defined as the rate of flow of energy reflected from a surface per unit solid angle from a unit source area on the surface.
Under direct sunlight there is a certain level of red and near infra-red light energy or irradiance that hits the earth's surface. Under cloudy conditions the amount decreases, with the decrease being most marked in the near infra-red region due to absorption by atmospheric moisture and clouds and due to shorter wave length scattering in the atmosphere. The energy incident on the detectors will change with changes in ambient conditions. Thus the approach of using energy (radiance) is subject to considerable variability that makes it difficult to calibrate or adopt standard decision making processes.
Reflectance is defined as the ratio of reflected energy to incident energy. Reflectance should stay the same or approximately the same under different light conditions for a given target. It is more consistent than radiance and hence easier to process so as to arrive at a machine decision. It is for this reason that reflectance has been chosen as the decision making criteria, rather than radiance.